The present invention relates to a multilayer adhesive tape based on a biaxially oriented polyolefin film, which during removal from a substrate is destroyed.
Adhesive tapes coated on one or both sides have been known for a long time. The first tapes had a support layer of cellulose hydrate. However, such tapes are highly sensitive to humidity or moisture content fluctuations. At low humidity values, the tape becomes brittle and tears easily, while at higher humidity values the rolls telescope, and the adhesive tape is difficult to tear off. Furthermore, the relatively aggressive adhesives attack to support film.
In view of these disadvantages, attempts were made subsequently to replace the cellulose hydrate support film by polymer foils or films. It was then discovered that, for example, adhesive tape with a support film of polyethylene terephthalate is too difficult to tear, while tapes based on cellulose acetate tear too easily. Tapes based on oriented polystyrene are too brittle and in addition have a low solvent resistance. Tapes with support foils of polyethylene, vinylidene chloride copolymers or plasticized polyvinyl chloride have little mechanical strength and are difficult to tear
These facts are described very clearly in U.S. Pat. No. 3,089,786, the object of which is to provide an adhesive tape based on polyvinyl chloride, which would not have the disadvantages of the known tapes. However, it is also known that tapes made on the basis of polyvinyl chloride are easily split under impact-like transverse stresses and tend to tear in the longitudinal direction. Attempts were therefore made to replace the polyvinyl chloride support films with other polymer support films, all the more so since environmental problems could be attributed to tapes based on polyvinyl chloride if destroyed by burning.
Adhesive synthetic plastic tapes based on polyolefins are also known, for example, from DE-OS No. 29 38 471.
Most of the references describing self-adhesive materials, adhesive strips or adhesive tapes concern either methods to improve their adhesive properties, for example, UK Patent Application No. 2,001,079, which is also directed to an especially decorative appearance of the products described therein, or with better separating properties of the adhesive layers from the protective layers attached to them, such as, for example, U.S. Pat. No. 4,339,485. A reduction of the risk of damage to surfaces by means of adhesive tape and a self-adhesive tape coated with adhesive on both sides serving this purpose, are also described in British Pat. No. 1,562,680.
There are, however, applications conceivable, in which an adhesive tape is needed which has such good adhesive properties that it either cannot be removed at all from the substrate to which it has been once applied, and/or it is destroyed by its forcible or unauthorized removal, thereby furnishing indications either by damage to the substrate or by fragments remaining thereon of its removal, and which cannot be reused under any circumstances, once it has been removed.
Examples where such security adhesive tapes may be used are price labels in self-service stores or official seals on automobile license plates, calibration marks on timepieces and counters, seal markings, etc.
For such purposes there are used at the present time paper stickers which are mechanically embossed or prestamped and thus contain predetermined breaking points, whereby they yield only small fragments corresponding to their stamping, upon unauthorized removal. These stickers have the disadvantage that, prior to their application, they must always be exposed to an additional work step, i.e., that of stamping, and that because of the intentional breaking points contained in them, they are difficult to draw, for example, from a supply roll, in one piece.